1. Field of the Invention
The present invention relates to an oscillation circuit and, more particularly, an oscillation circuit suitable for use as a local oscillator of an upper heterodyne receiver.
2. Description of the Related Art
For example, in the case of frequency-converting a television signal in a band of tens MHz to hundreds MHz to a television intermediate frequency signal, the television signal is once frequency-converted to a first intermediate frequency signal in a high band of 1300 MHz. At this time, a local oscillator oscillates at a frequency higher than 1300 MHz.
A conventional oscillation circuit used as such a local oscillator is shown in FIG. 5. The oscillation circuit is of a common collector type. The collector of an oscillation transistor 31 to which a source voltage (E) is applied is connected to the ground in a high frequency manner by a d.c. blocking capacitor 32 and feedback capacitors 33 and 34 are connected between the base and emitter and between the emitter and collector, respectively. Inductance means 35 is connected between the base and the ground. The inductance means 35 is connected in parallel to the feedback capacitors 33 and 34 which are connected in series in a high frequency manner and becomes equivalently inductive at an oscillation frequency.
In the inductance means 35, a clap capacitor 36, a varactor diode 37, and an inductor 38 are connected in series. An oscillation frequency is determined by a parallel resonance frequency obtained by an equivalent inductance of the inductance means 35 and a series capacity of the feedback capacitors 33 and 34 which are connected in series.
A resistor 39 is connected between the emitter of the oscillator transistor 31 and the ground and a resistor 40 is connected between the base and the ground. A source voltage (E) is supplied to the base via a resistor 41. The resistor 39 serves as an emitter-bias resistor for supplying a bias voltage to the emitter. The resistors 40 and 41 serve as base-bias resistors for applying a bias voltage to the base.
An oscillation signal outputted from the emitter of the oscillation transistor 31 is supplied to an amplifier 43 via a pickup capacitor 42 and supplied to a mixing circuit (not shown) from the amplifier 43.
At this time, a control voltage V is applied to the cathode of the varactor diode 37 by a choke coil 44. The capacity of the varactor diode 37 is changed by changing the voltage V, thereby varying the oscillation frequency in correspondence with the frequency of the television signal to be received.
Specifically, the capacity of the varactor diode 37 decreases as the control voltage V applied to the varactor diode 37 is made higher, an inductance equivalent value of the inductance means 35 decreases and the oscillation frequency becomes higher.
As the control voltage V applied to the varactor diode 37 is decreased, the capacity of the varactor diode 37 increases, an inductance equivalent value of the inductance means 35 increases, and the oscillation frequency becomes lower.
Generally, the level of the oscillation signal outputted from the oscillation circuit becomes higher as the oscillation frequency is lowered. Noises are also produced from the oscillation circuit and the level of the noises also becomes higher as the frequency is lowered.
When the oscillation circuit is used as a local oscillation circuit of an upper heterodyne receiver, high-level noise in a first intermediate frequency band which is lower than the oscillation frequency is carried to a mixing circuit. Consequently, there is a drawback that the S/N ratio of the first intermediate frequency signal deteriorates.